Before I explain what's going on, I need to ask: what does a truck's alarm sound like in real life? I've heard car alarms outside but never an alarm coming from a semi truck.

Anyway, this is how it works:

• Fill clocks. Again. If you don't know what that is, it's an extremely fast redstone clock that can run command blocks 20 times every second. To make a fill clock, you'll need two command blocks. Place of them down and enter this command: /fill ~ ~1 ~ ~10 ~1 ~ stone. Now, put a block above the command block you just placed and put down another command block on top with this command: /fill ~ ~-1 ~ ~10 ~-1 ~ redstone_block. Of course, the size of the fill clock is up to you. Now, place a redstone block in between the two command blocks and congratulations, you just created a fill clock!
• The clock tests for a block of TNT at a specific set of coords 20 times every second. If the game detects TNT at those coordinates (which is how it'll be by default in this custom map), a comparator output is released and inverts another signal. This is known as a NOT gate, where if the input is off, the output is on, and if the input is on, the output is off.
• If the TNT is detonated by a flaming arrow, then obviously it isn't there anymore. Because of this, the comparator stops giving away a signal and therefore the redstone torch of the NOT gate releases a signal of its own. This is where things start to get sort of complex.
• Because I'm totally an expert at organizing command block creations, I was forced to make the output a single command block, which places a redstone block that activates other command blocks.
• There was a lot of trial and error involved in the timing and the motion values of the two FallingSand entities involved. But if anyone is curious, these are the summon command for them:
  • Bucket: /summon FallingSand 6 131 -32 {Time:1,Block:"flower_pot",Motion:[0.55,2.0,-0.06]}
  • Anvil: /summon FallingSand 25 141 -35 {Time:1,Block:"anvil",Data:7,Motion:[-0.3,0.2,-0.2]}
• As demonstrated in the gfy, the "bucket" falls onto a block of TNT, which ignites it and prompts it to "explode". This second "explosion" in turn sends an anvil flying. Why the quotation marks? There is no real explosion. The command, /particle hugeexplosion 27 141 -35 0 0 0 0 1 force is used to fake it. The reason why I didn't use a real one is because when I tried that before, I noticed that the Falling Anvil got propelled from the explosion and went a lot further than it should have.
• A tellraw command (/tellraw @a {"text":"","extra":[{"text":"[Truck alarm!]","color":"dark_red","italic":"true"}]}) displays a chat message that reads "[Truck alarm!]" in dark red characters.
• A horde and a boss zombie is alerted due to the annoying sounds of the alarm the truck is making. But to the survivors' (the players) surprise, it turns out the aforementioned boss zombie was stuck inside the cargo of the truck the whole time! With an endless supply of anger and a state of unbridled rage, it busts out of its prison and goes after the survivors. The horde isn't seen in the gfy, but it's real, I'm telling you. The command to summon the boss is: /summon Zombie 16 131 -44 {CustomName:"Tank",CustomNameVisible:1,IsVillager:1,CanBreakDoors:1,Equipment:[{id:diamond_sword,Count:1,tag:{ench:[{id:19,lvl:3}]}},{id:iron_boots,Count:1,tag:{ench:[{id:8,lvl:3},{id:34,lvl:1}]}},{id:iron_leggings,Count:1,tag:{ench:[{id:34,lvl:1}]}},{id:iron_chestplate,Count:1,tag:{ench:[{id:34,lvl:1}]}},{id:iron_helmet,Count:1,tag:{ench:[{id:34,lvl:1}]}}],Attributes:[{Name:generic.maxHealth,Base:400},{Name:generic.movementSpeed,Base:0.3},{Name:generic.attackDamage,Base:79},{Name:generic.knockbackResistance,Base:1.0}],PersistenceRequired:1}.
• The blocks that come flying out of the truck's cargo are obviously FallingSand entities. I'm sure I don't have to go over every single one of them. There's two particle commands to go along with the effect:
  • /particle iconcrack_155 15 131 -44 0 0 0 0.3 650 force
  • /particle explode 15 131 -44 0 0 0 0.5 100 force
• This is sort of off-topic, but I thought it was worth mentioning. From the looks of it, a bucket landing on a block of TNT shouldn't ignited it right away, right? Yes, but while it wasn't seen in the gfy, there is actually a fire on the roof. The bucket lands into the patch of flames, then bounces and comes in contact with the TNT. And we all know that fire and TNT hate each other.

Also:

• I did not post this as an attempt to advertise my map or anybody else's map.
• If you would like to implement this idea into your own custom map, feel free to do so without giving me any credit.